Hello out there,
There's a bug in SimpleGraph. It is easy to reproduce in SGDemo:
1. Insert a node
2. Now open the properties window of this node
3. Change the node's shape (Elliptic for example)
4. Click 'Clear Background' (It's unimportant wether the node contains a background image or not)
5. Now click 'Apply changes' and you'll get an exception
Best regards, P_G